This invention relates to steam turbines and in particular to double flow steam turbines.
In general, steam turbines operate to convert energy stored in high-pressure, high-temperature steam into rotational mechanical movement. The steam turbines employed by electric utilities in the generation of electric power, typically comprise a plurality of turbine buckets radially mounted on a rotor shaft and disposed so as to form a plurality of bucket wheels. The rotor shaft, with its bucket wheels, is mounted on bearings with the bucket wheels disposed inside an inner shell which is in turn surrounded by an outer shell. These double shells serve the function of forming a pressurizable housing in which the bucket wheels rotate and of preventing potentially damaging thermal gradients. The bucket wheels are disposed between stationary nozzle rings. These nozzle rings are formed by circular arrays of stationary curved partitions. These partitions are generally referred to as nozzle partitions and the spaces between the partitions as nozzles. As steam passes through the pressurizable inner shell it alternately passes through sequences of stationary nozzle partitions and rotating turbine bucket wheels to produce rotational movement of the rotor shaft. These concepts are elementary and are generally well known in the turbine arts.
Modern large steam turbines generally comprise several sections such as high-pressure section, intermediate pressure section and low-pressure sections. These sections possess various design characteristics so as to permit the extraction of the largest possible amount of energy from the expansion of steam through the turbine sections. It is a common practice to have one or more of these sections configured in a double flow arrangement, in which steam entering a middle portion of the section encounters a diverging flow path. Following entry into this middle portion of one of the turbine sections, the steam exits in opposite directions with both flows directed to rotate the turbine shaft in the same direction. Thus for example, steam entering from the top or bottom exits toward the left and right. This double flow configuration contributes to the overall machine efficiency.
One of the important parts of a double flow turbine section is the inner web of the diaphragm. Before the steam flowing in opposite directions encounters any turbine bucket wheels, it encounters a first set of nozzle partitions which direct the steam against the turbine buckets at optimal angles. There are two sets of these nozzle partitions, each arranged in an annular spoked pattern on opposite sides of the middle (that is, steam entrance) portion of the double flow turbine diaphragm. Along their radially outward tips, these partitions are affixed to, as by welding, outer annular rings which are fitted into recesses within the inner turbine shell. Of greater interest in the present invention, however, is the fact that along the radially inner portion of these nozzle partitions, they are affixed to the inner web of the diaphragm, again typically by welding. Thus this inner web has two sets of annularly configured nozzle partitions affixed to it (one on each end of the diaphragm). Its primary function is to support this particular set of nozzle partitions. These are the first nozzle-defining partitions encountered in the steam flow path of the particular double flow section under consideration. The remaining nozzle partitions are disposed between the rotating turbine bucket wheels using differently configured diaphragms. In addition to supporting these first rings of nozzle partitions, the inner web also serves another important function, in that it significantly helps to define the steam flow path. In particular it prevents direct contact between the incoming steam and the rotor shaft. The design of this web ensures that the entire steam flow is directed between the nozzle partitions and thence to the turbine bucket wheels.
In previous designs of this inner web, single piece fabrication was employed. However, this design can be difficult to implement in practice. In particular it may be difficult to keep two individual steam path assemblies flat, parallel, circumferentially aligned and properly axially spaced during the thermal distortions inherent during weld fabrication. Moreover, during machining, both ends of the web have to be concentrically aligned, joint pitches must be aligned, and steam paths machined separately and leveled. As a result of accumulated variances in machining, assembly and distortion in welding, rework is often required to assure that all dimensions are satisfactory. In one proposed solution to this problem the inner web is fabricated in two axial pieces bolted together at the midline. While being an improvement over the single piece design for low-temperature applications, this bolted-together-web design is undesirable in high-temperature applications. In addition, neither the single piece nor the bolted-together design permit relative axial movement due to transient or steady-state thermal expansion forces.